1. Field of the Invention
The present invention relates to a transmission power control apparatus used for a transmission power control in a CDMA (code division multiple access) radio communication apparatus, for example, and a transmission power control unit used in such a transmission power control apparatus.
2. Description of the Related Art
For example, in a CDMA radio communication apparatus, the transmission power is controlled in accordance with the reception power.
FIG. 6 shows the configuration of a conventional radio transmitter having a function of performing such a transmission power control.
The radio transmitter is composed of an AGC amplifier 1, a power amplifier 2, and an antenna 3.
In a data transmitting operation, the AGC amplifier 1 and the power amplifier 2 are activated in response to a burst-on signal as a transmission request. A transmission signal is amplified by the AGC amplifier 1 and the power amplifier 2 and then supplied to the antenna 3. The transmission signal is transmitted from the antenna 3.
In the above radio transmitter, the transmission power control is performed in such a manner that the gain of the AGC amplifier 1 is varied in accordance with a power control signal that is generated through a calculation that is performed by a receiving section (not shown) based on the reception power. The gain of the power amplifier 2 is fixed. However, since the AGC amplifier 1 can provide a negative gain, a total gain that is lower than the gain of the power amplifier 2 can be obtained.
For example, if the gain of the power amplifier 2 is 30 dB and the dynamic range Of the gain of the AGC amplifier 1 is -20 to 50 dB, the dynamic range of the total gain of the radio transmitter is 10-80 dB.
FIG. 7 shows graphs showing relationships between the reception power and the gain of an AGC amplifier 1, the gain of a power amplifier 2, and the total gain in the radio transmitter of FIG. 6. As shown in FIG. 7, when the necessary total gain is relative low (i.e. the reception power is low level), the gain of the AGC amplifier 1 is negative because the gain of a power amplifier 2 is fixed.
FIG. 8 shows a relationship between the transmission power and the reception power in the above radio transmitter. In a range where the transmission power is relatively high, the power consumption decreases as the transmission power decreases. However, in a range where the transmission power is lower than a certain value, almost no decrease occurs in the power consumption even if the transmission power decreases.
The above fact means that unduly high power is consumed in the range where the transmission power is low. This is not preferable particularly in mobile communication apparatus (e.g. cellular telephone) or the like that use a battery whose capacity is limited. It causes the reduction of available speech time. Particularly, if the mobile communication apparatus is used as the CDMA radio communication apparatus (e.g. CDMA cellular telephone), this is very serious problem because the CDMA system requires wider dynamic range than other digital system like PDC (Japanese Personal Digital Cellular). So, if the consumed power in the range where the transmission is low becomes reduced, the available speech time is expected to be much longer.
As described above, conventionally, the transmission power is controlled by adjusting the gain of a variable gain amplifier while a certain gain is secured by a fixed gain amplifier. This causes a problem that a decrease in transmission power causes almost no decrease in power consumption in a range where the transmission power is lower than a certain value. It means that the power utilization efficiency is low.